Exhaust gas recirculation (EGR) valves are employed in connection with internal combustion engines to aid in the lowering of regulated emissions and to enhance fuel economy by metering exhaust gas to the intake manifold for delivery to the combustion chamber. In the exhaust gas recirculation valve assembly set forth in U.S. Pat. No. 5,020,505 issued Jun. 4, 1991, to Grey et al., a base assembly contains a valve member in engagement with a valve seat. The base supports an actuator assembly including a linear, electromagnetic solenoid actuator which is operable to move the valve member relative to the valve seat to regulate the flow of exhaust gas therethrough. Critical to the operation of the disclosed EGR valve is the bearing function which provides both guidance and sealing against leakage of gasses in or out of the base. Fixed-type bearings such as are disclosed in the above referenced device may often place considerable frictional loads on the valve stem requiring the use of a larger than optimum actuator in order to overcome the resultant parasitic load. Larger actuators may, in turn, incur mass, power consumption, installation, durability, cost and robustness penalties.
The desirability of a valve stem seal having minimum leakage may run counter to common design criteria for a journal-type bearing which typically will require appreciable clearance at the valve stem/bearing interface in order to limit binding between components. Such clearance, however, will result in added leakage at the interface conflicting with the desire to limit emissions of untreated engine exhaust. In such applications, a compromise is reached where a maximum leakage quantity is tolerated in order to minimize friction between the bearing and the valve stem. Such acceptable leakage rates may be conducive to actuator corrosion caused by condensation of highly corrosive exhaust gas escaping through the bearing seal. Additionally, in an environment of ever-tightening emission regulations for internal combustion engines, previously acceptable leakage rates become unacceptably high, further limiting the bearing/valve stem tolerances.
Optimum design criteria dictates that, to preclude carbon encrustation on the valve member and detrimental condensation of exhaust gases within the actuator, the valve member should open outwardly, away from its seat and into the exhaust flow, with exhaust backpressure on its lower face and manifold pressure on its upper face. However, such an arrangement is presently not permitted due to undesirable amounts of unmetered air ingested by the engine due to leakage of external, unmetered air past the bearing seal caused by the relatively large resultant pressure differential between the manifold and the atmosphere. Such leakage adversely impacts engine performance, emissions, and fuel economy.